Optical semiconductor devices are found in many common appliances, such as digital cameras, digital camcorders, laptop computers, cellular phones, and many other devices. Generally, optical devices comprise charge coupled devices along with an image or video processor to compress and transmit data.
FIG. 1 shows optical semiconductor device 10 in current practice. The device 10 is a leadframe based device wherein an optical die 11 is mounted on a leadframe 12. The leadframe 12 is partially encased in a mold compound 13 leaving an opening 14 for the optical die 11. Optionally, a light permeable covering (not shown) is placed over the optical die 11 for protection. Although such devices 10 offer a high degree of reliability, they are generally limited to a low input output (I/O) count. As devices with optical die 11 increase in complexity and consumers demand items such as cameras having more image capturing capability, die sizes and I/O counts increase. In such applications with dozens or hundreds of I/O, such devices 10 are not an option. In one current example, every prominent manufacturer of high definition televisions offers a rear projection display option. This is commonly marketed by Sony as SXRD and Samsung as DLP, licensed from Texas Instruments. The complexity of such an optical application requires dozens to hundreds of I/O.
To overcome the issues mentioned above, the semiconductor industry has moved toward Ball Grid Array (BGA) packages. The BGA is descended from the pin grid array (PGA), which is a package with one face covered (or partly covered) with pins in a grid pattern. These pins are used to conduct electrical signals from the integrated circuit (IC) to the printed circuit board (PCB) it is placed on. In a BGA, the pins are replaced by balls of solder stuck to the bottom of the package. The device is placed on a PCB having copper pads in a pattern that matches the solder balls. The assembly is then heated, either in a reflow oven or by an infrared heater, causing the solder balls to melt. Surface tension causes the molten solder to hold the package in alignment with the circuit board, at the correct separation distance, while the solder cools and solidifies. The BGA is a solution to the problem of producing a miniature package for an IC with many hundreds of I/O. As pin grid arrays and dual-in-line (DIP) surface mount (SOIC) packages are produced with more and more pins, and with decreasing spacing between the pins, difficulties arose in the soldering process. As package pins got closer together, the danger of accidentally bridging adjacent pins with solder grew. BGAs do not have this problem, because the solder is factory-applied to the package in exactly the right amount. Alternatively, solder balls are able to be replaced by solder landing pads, forming a Land Grid Array (LGA) package.
FIG. 2 shows a cutaway image of a generic BGA package 20. Generally, an IC 21 has bondpads 22 to which bondwires 23 are affixed. The IC 21 is mounted on a substrate 24. In current practice, the substrate 24 is a laminate, such as polyimide. Generally, the substrate 24 is of a similar construction to a PCB. The substrate 24 has copper patterns 25 formed thereon. The bondwires 23 effectuate electrical contact between the IC 21 and the copper patterns 25. The copper patterns 25 are electrically connected to solder balls 26 through via holes 27 in the substrate 24. In most embodiments of BGA packages, the IC 21 is encapsulated by a mold compound 28. In optical applications, an opening 29 is formed over the IC 21. Optionally, a light permeable covering, such as glass, is mounted in the opening 29 to protect the die 21. Although BGA packages effectuate large I/O count devices in small areas, they are susceptible to moisture. Generally, moisture seeps into packages while awaiting assembly into a finished product, such as a computer. When the package is quickly heated to solder the device into its end application, moisture trapped within the device turns into vapor and cannot escape quickly enough, causing the package to burst open. This phenomenon is known as the “popcorn” effect. What is needed is a semiconductor package that is robust to both structural stressors and moisture.